U.S. Pat. No. 4,017,318 discloses two basic methods for preparing photosensitive colored glasses or polychromatic glasses, as they have more recently been termed, each method being founded in a sequence of irradiation and heat treating steps. The glasses described in that patent evidence a wide variation in base compositions but each requires the presence of silver, an alkali metal oxide which is preferably Na.sub.2 O, fluoride, and at least one other halide selected from the group of chloride, bromide, and iodide. The glasses are subjected to high energy or actinic radiations selected from the group of high velocity electrons, X-radiations, and ultra-violet radiations in the range of about 2800A-3500A. The heat treatments comprehend exposures to temperatures between about the transformation range of the glass up to about the softening point thereof. Where ultra-violet radiation constitutes the effective actinic radiation, CeO.sub.2 is recited as being a necessary constituent of the glass composition.
In the first of the two methods described, the glass is initially irradiated with high energy or actinic radiations to cause the development of a latent image in the glass. The duration of this exposure and the flux thereof, i.e., the energy/unit area of the irradiation, determine the final color which will be exhibited by the glass. Thereafter, the glass is exposed to a heat treatment at a temperature between about the transformation range and the softening point of the glass to effect the precipitation in situ of colloidal silver particles which act as nuclei.
Where a transparent colored glass is desired, this heat treatment will be conducted only for so long as to cause the precipitation of colloidal silver nuclei with the possible growth thereon of extremely small microcrystals of alkali metal fluoride-silver halide, e.g., NaF + (AgCl and/or AgBr and/or AgI). Where an opal glass is to be produced, the heat treatment will be prolonged for a sufficient length of time and at a sufficiently high temperature not only to promote the precipitation in situ of colloidal silver nuclei, but also to cause the growth of said microcrystals on the silver nuclei to a great enough size to scatter light.
Subsequently, the nucleated glass is cooled, customarily to room temperature but, in any event, to a temperature at least 25.degree. C. below the strain point of the glass, and re-exposed to high energy or actinic radiation. This second exposure process acts to develop the color, the hue of which was determined by the previous exposure. Finally, the glass is again heated to a temperature between about the transformation range and the softening point of the glass to bring out the desired color therein.
Whereas the mechanism of color production was not fully understood, it was believed that the quantity of silver precipitated and the geometry of the precipitated particles, as well as perhaps the refractive index of any crystals developed, decide the color exhibited. Nevertheless, because the colors can be attained with very minor amounts of silver, it was postulated that at least one of the following situations obtained: (1) the presence of discrete colloidal particles of silver less than about 200A in the smallest dimension; (2) metallic silver was deposited within alkali fluoride-silver halide microcrystals, the silver-containing portion of the microcrystals being less than about 200A in the smallest dimension; and (3) metallic silver was deposited upon the surface of the microcrystals, the silver-coated portion of the microcrystals being less than about 200A in the smallest dimension. The microcrystals are present in a concentration of at least about 0.005% by volume.
The patent noted that the use of consecutive or interrupted heat treatments, either after the initial irradiation to high energy or actinic radiation or after the second irradiation step, can be useful in intensifying the final color produced. Consequently, although the reaction mechanism underlying that phenomenon is not completely comprehended, experience indicated that two or more heat treatments at temperatures between the transformation range and the softening point of the glass do not alter the color developed, but can promote a more vivid color than a single heat treatment of equal or longer duration.
The patent also observed that the identity of the color developed in the glass was depended upon the duration and flux of the initial exposure to high energy or actinic radiation. Thus, the least exposure yielded a green coloration followed by blue, violet, red, orange, and yellow as the exposure time and/or flux was increased.
The second general method for preparing photosensitive colored glass disclosed in U.S. Pat. No. 4,017,318, supra, involves the production of glass articles exhibiting a single color, but which color can be varied over the full range of the visible spectrum. Such glasses were formed from compositions wherein the silver content was partially thermoreduced in a heat treating step at temperatures between the transformation range and the softening point of the glass without a previous irradiation by high energy or actinic radiation. This heat treatment can conveniently be conducted during the customary annealing of the initially formed article. Subsequently, the so-conditioned or presensitized glass is subjected to high energy or actinic radiation followed by heat treatment at temperatures between the transformation range and the softening point of the glass.
The monochrome color produced is dependent upon the concentrations of silver and the thermoreducing agent included in the glass composition. SnO is stated to be the preferred agent for that purpose. The color displayed by the glass progressively changed from green through blue, violet, red, orange, and yellow with increased amounts of thermoreducing agent where the silver concentration is held constant.
As is evident, this latter method eliminates the need for the first exposure to high energy or actinic radiation but has the disadvantage of permitting the development of only one color in a given article of glass, since the initial thermal reduction determines the final color to be produced. The subsequent exposure and heat treatment merely bring out that color.
United States application Ser. No. 778,160, filed Mar. 16, 1977 by Joseph Ference, describes an improvement upon the method for producing polychromatic glasses disclosed in U.S. Pat. No. 4,017,318, supra, wherein the time required for developing the color is shortened and the colors, themselves, are often more vivid.
The preferred embodiment of the invention contemplates four basic steps:
(1) a glass article is formed having a composition coming within the ranges set out in U.S. Pat. No. 4,017,318;
(2) the glass article is exposed to high energy or actinic radiation for a sufficient length of time to develop a latent image therein;
(3) the high energy or actinic radiation is removed and the glass article heated to a temperature between the transformation range and the softening point of the glass for a sufficient length of time to cause nucleation and growth of microcrystals consisting of alkali fluoride containing at least one silver halide selected from the group of AgCl, AgBr, and AgI; and then
(4) the glass article is re-exposed to high energy or actinic radiation while at a temperature between about 200.degree.-410.degree. C. for a sufficient length of time to cause metallic silver to be deposited as discrete colloidal particles less than about 200A in the smallest dimension, and/or deposited on the surface of said microcrystals, the portion of the microcrystal coated with silver being less than about 200A in the smallest dimension, and/or deposited within said microcrystals, the silver-containing part of the microcrystal being less than about 200A in the smallest dimension, said microcrystals having a concentration of at least 0.005% by volume.
Where desired, the initial irradiation by high energy or actinic radiation may also be undertaken at temperatures between about 200.degree.-410.degree. C. That practice is optional, however, since it does not appear to improve significantly the intensity of the final color developed within the glass, although it does have the advantage of reducing the time required for nucleation and incipient crystallization. Moreover, should this initial irradiation of the glass at elevated temperatures be prolonged for an extended period of time, the glass will take on a permanent yellowish cast. Such a phenomenon is, of course, unwanted where a spectrum of colors is desired.
In summary, both U.S. Pat. No. 4,017,318 and U.S. application Ser. No. 778,160 teach that, where the development of a variety of colors is desired in polychromatic glasses, two exposures to high energy or actinic radiation are demanded. In actual practice, the first irradiation treatment is commonly of relatively short duration, e.g., a few minutes will frequently be sufficient, whereas the second exposure is of much longer duration, i.e., typically one hour or longer even when combined with the second heat treatment as disclosed in U.S. application Ser. No. 778,160. It can readily be appreciated that the energy required for the exposure step, i.e., the energy required for the high intensity radiation for an extended period of time, adds a very substantial factor to the cost of the finished article. Furthermore, the need for irradiation places constraints on the sizes of articles that can be so-treated and the overall speed of production.
Accordingly, it would be highly desirable if polychromatic glass articles could be produced exhibiting a variety of colors of high intensity without the requirement of a lengthy irradiation step.